Change-over valve for boom cylinder of excavating/slewing work truck

ABSTRACT

In an excavating/slewing work truck, the boom falls freely at the time of lowering operation without requiring any power but the flow rate of a pump increases excessively when the speed is balanced with other acutuators and power loss is inevitable for enhancing the operability. In order to eliminate this inconvenience, a first oil path ( 41 ) connecting a bottom side cylinder port (CB) and a tank port (T 2 ), a second oil path ( 42 ) connecting a pump port (P 2 ) and a rod side cylinder port (CR), and a third oil path ( 43 ) connecting a pump port (P 1 ) and a tank port (T 1 ) are provided, respectively, with first, second and third restrictors ( 61 ), ( 62 ) and ( 63 ) at the boom down position of a change-over valve ( 51 ) for the boom cylinder of an excavating/slewing work truck, wherein the first restrictor ( 61 ) restricts by such an amount as the work machine lowers gravitationally an the second restrictor ( 62 ) restricts by such an amount as the pressure on the boom side is not exceeded.

TECHNICAL FIELD

[0001] The invention relates to a change-over valve for rotationaloperation of a boom of a working implement of an excavating/slewing worktruck, the change-over valve being improved to reduce power loss of thehydraulic pump.

BACKGROUND ART

[0002] A conventional excavating/slewing work truck is provided at adriver's seat thereof with operation levers for rotating parts of aworking machine such as a boom, an arm and a bucket. Each of the leversis connected directly or through a pilot valve to a spool of acorresponding change-over valve so as to slide the spool for changingthe change-over valve.

[0003] As shown in FIG. 14, a change-over valve for a boom cylinder 23is a three-position pilot change-over valve having six ports. When thecorresponding operation lever is rotated from a neutral position to aboom down position, open areas of three oil paths in the change-overvalve are gradually changed so as to make a speed-control duringmovement of the spool from its neutral position to its full strokeposition.

[0004] The areas of the three oil paths are shown in FIG. 12. When thespool is in the neutral position, a third oil path 43 is opened for freepassage so as to connect a first pump port P1 to a tank port T1, while aboom cylinder bottom side port (hereinafter, referred to as “bottomport”) CB, a boom cylinder rod side port (hereinafter, referred to as“rod port”) CR, a second pump port P2 and a second tank port T2 areblocked.

[0005] While the spool is moved for lowering the boom from the neutralposition, the open area (c) of the third oil path 43 is rapidlyrestricted at an early period of the movement of spool, then it isgradually restricted, and finally the third oil path 43 is perfectlyclosed when the spool reaches its full stroke position.

[0006] During the same movement of spool, the open area (a) of a firstoil path 41 for connecting the bottom port CB to the second tank port T2and the open area (b) of a second oil path 42 for connecting the secondpump port P2 to the rod port CR are gradually opened. The open area (a)of the first oil path 41 is open but restricted to some degree when thespool reaches its full stroke. The open area (b) of the second oil path42 is rapidly widened just before the spool reaches the full stroke, soas to be larger than the open area (a) of the first oil path 41.

[0007] While the boom is lowered, the lowered boom is gravitationallyaccelerated in an early period between times t1 and t2, as shown in FIG.13, however, an amount of oil supplied from the second oil path isincreased. However, when a time t3 is reached, the increased oilsupplied from the second path becomes such a level as to suddenlyincrease the lowering speed, thereby causing a shock.

[0008] Therefore, the first oil path is restricted so as to be narrowerthan the second oil path.

[0009] However, consequently, the pressure on the bottom side of thecylinder is increased so as to increase the pressure on the rod sidethereof and the pump pressure, thereby causing a large power loss.

[0010] Such a power loss may be reduced by a conventional art disclosedin Japanese Laid Open Gazette No. Hei 10-89317, wherein pressure in adrain oil passage to a tank is detected for adjusting the amount of oildischarged from a pump so that oil discharged from the pump is reducedwhen a boom is lowered. However, this art requires a complicatedhydraulic circuit and an expensive variable displacement hydraulic pump.

DISCLOSURE OF THE INVENTION

[0011] In a change-over valve for a boom cylinder of anexcavating/slewing work truck according to the present invention, whenthe change-over valve is set at its boom down position, a first oil pathconnects a bottom side cylinder port to a tank port, a second oil pathconnects a pump port to a rod side cylinder port, a third oil pathconnects another pump port to another tank port, and the first, secondand third oil paths are provided with respective first, second and thirdrestrictors. The first restrictor restricts by such an amount as togravitationally lower a work machine, and the second restrictorrestricts by such an amount as not to exceed pressure on the bottomside. Therefore, while a boom is gravitationally lowered, oil pressurefor lowering the boom is not applied, thereby preventing the loweredboom from being shocked.

[0012] According to the present invention, the third restrictorrestricts by such an amount as to adapt an excavating/slewing work truckto be jacked up when an engine of the work truck idles, i.e. to beeconomically jacked up while reducing power loss.

[0013] According to the present invention, the third oil path isconnected to a tank oil passage provided with a breed amount change-overvalve, which automatically change the breed amount without manualoperation in correspondence to a kind of work when the boom is operated.

[0014] According to the present invention, the breed amount change-overvalve is provided in a spool of the change-over valve for a boom so thatthe breed amount is automatically changed without manual operation incorrespondence to a kind of work when the boom is operated.

BRIEF DESCRIPTION OF THE DRAWING

[0015]FIG. 1 is an entire side view of an excavating/slewing work truckequipped with a change-over valve according to the present invention.

[0016]FIG. 2 is a hydraulic oil circuit diagram of a hydraulic drivedevice according to the present invention.

[0017]FIG. 3 is an enlarged hydraulic oil circuit diagram of achange-over valve for a boom.

[0018]FIG. 4 is a graph showing open areas of oil paths among ports inthe change-over valve relative to a stroke for lowering the boom.

[0019]FIG. 5 is a graph showing hydraulic pressures in the change-overvalve relative to a time for lowering the boom.

[0020]FIG. 6 is a view of the excavating/slewing work truck when it isjacked up.

[0021]FIG. 7 is a hydraulic oil circuit diagram according to anembodiment wherein a breed amount change-over valve is provided in atank oil passage.

[0022]FIG. 8 is a hydraulic oil circuit diagram when the breed amountchange-over valve is provided in a spool of the change-over valve forthe boom.

[0023]FIG. 9 is a sectional view of the breed amount change-over valveprovided in the spool of the change-over valve for the boom.

[0024]FIG. 10 is a sectional view of the same when the boom is lowered.

[0025]FIG. 11 is a sectional view of the same when the lowered boomreaches its full stroke.

[0026]FIG. 12 is a graph showing open areas of oil paths among ports inthe conventional change-over valve relative to a stroke for lowering theboom.

[0027]FIG. 13 is a graph showing hydraulic pressures in the conventionalchange-over valve relative to a time for lowering the boom.

[0028]FIG. 14 is a hydraulic circuit diagram of the conventionalchange-over valve for the boom.

BEST MODE FOR CARRYING OUT THE INVENTION

[0029] A preferred embodiment of the invention will be described inaccordance with the accompanying drawings.

[0030] First, description will be given of a general structure of anexcavating/slewing work truck according to the present invention.

[0031] As shown in FIG. 1, the excavating/slewing work truck has acrawler-type traveling device 1. A slewing frame 8 is rotatablysupported through a vertical pivotal bearing 7 on the center of thetraveling device 1. A blade 10 is vertically movably disposed on one offront and rear portions of the traveling device 1.

[0032] A bonnet 9 covering an engine and so on is disposed on a rearportion of the slewing frame 8. A cabin 22 enclosing a driver's portionis disposed in front of the bonnet 9.

[0033] A working machine 2 is provided on a front end portion of theslewing frame 8. With respect to the working machine 2, a boom bracket12 is laterally rotatably attached onto the front end portion of theslewing frame 8. The boom bracket 12 longitudinally rotatably supports alower end portion of a boom 6. The boom 6 is doglegged forward as beingapparent when viewed in side. An arm 6 is rotatably supported on theother end portion of the boom 6 so as to rotatably support at a tipthereof a bucket 4 serving as a working attachment.

[0034] A boom cylinder bracket 25 is provided on an intermediate frontsurface of the boom 6. A boom cylinder 23 is interposed between the boombracket 12 and the boom cylinder bracket 25. An arm cylinder bottombracket 26 is provided on an intermediate back surface of the boom 6. Abucket cylinder bracket 27 is provided on a base end portion of the arm5. An arm cylinder 29 is interposed between the arm cylinder bottombracket 26 and the bucket cylinder bracket 27. The bucket cylinder 24 isinterposed between the bucket cylinder bracket 27 and a stay 11connected to the bucket 4.

[0035] Accordingly, the boom cylinder 23 acts to rotate the boom 6, thearm cylinder 29 the arm 5, and the bucket cylinder 24 the bucket 4. Theboom cylinder 23, the arm cylinder 29 and the bucket cylinder 24 arehydraulic cylinders, which are telescoped by supply of pressure oil fromhydraulic pumps through respective change-over valves that are switchedby operating respective levers provided above the valves.

[0036] A swing cylinder 17 is disposed in a side portion of the slewingframe 8 and pivotally supported at a base portion thereof by the slewingframe 8. A tip of a cylinder rod of the swing cylinder 17 is connectedto the boom bracket 12. The swing cylinder 17 acts to laterally rotatethe slewing frame 8 relative to the boom bracket 12 so as to laterallyrotate the working machine 2.

[0037] A hydraulic motor 13 (see FIG. 2) is provided on a top portion ofthe slewing frame bearing 7 so as to act to laterally rotate the slewingframe 8 in all directions. A blade cylinder 14 is interposed between atrack frame 3 of the traveling device 1 and the blade 10 so as tovertically move the blade 10. Traveling hydraulic motors 15R and 15L areprovided on inward sides of respective drive sprockets on one of frontand rear portions of the track frame 3 so as to enable the crawler-typetraveling device 1.

[0038] Description will now be given of a hydraulic circuit of theexcavating/slewing work truck including the hydraulic cylinders andmotors serving as hydraulic actuators in accordance with FIG. 2.

[0039] A first hydraulic pump 91, a second hydraulic pump 92, and athird hydraulic port P3 are connected in parallel to an output shaft ofthe engine disposed in the bonnet 9. Output hydraulic paths of the firstand second hydraulic pumps 91 and 92 are led through a change-over valve20 which is opened by action of the hydraulic pumps. A first center oilpath 31 serving as an output path of the first hydraulic pump 91 isconnected in parallel with a relief valve 35 for setting the outputhydraulic pressure therein. On the first center oil path 31 areconnected a traveling hydraulic motor change-over valve 50R, a boomchange-over valve 51 and a bucket change-over valve 52 in tandem. Thetraveling hydraulic motor change-over valve 5OR is provided to changeoil supply for the traveling hydraulic motor 15R provided on one ofright and left sides (the right side in this embodiment). The boomchange-over valve 51 is provided to change oil supply for the boomcylinder 23, and the bucket change-over valve 52 for the bucket cylinder24.

[0040] A second center oil path 32 serving as an output path of thesecond hydraulic pump 92 is connected in parallel with a relief valve 36for setting the output hydraulic pressure therein. On the second centeroil path 32 are connected a traveling hydraulic motor change-over valve50L, a swing change-over valve 58, an arm change-over valve 55 and a PTOchange-over valve 53 in tandem. An output path of a third hydraulic pump93 is provided thereon with a slewing change-over valve 54 and a bladechange-over valve 53 connected in tandem and is set about outputhydraulic pressure therein by a relief valve 37.

[0041] Description will be given of a structure of the boom change-overvalve 51 for controlling the telescoping action of the boom cylinder 23,serving as a principal portion of the invention.

[0042] As shown in FIG. 3, when the boom change-over valve 51 is set inneutral, an oil path between a first pump port P1 and a first tank portT1 is opened for free passage. A bottom port CB, a rod port CR, a secondpump port P2 and a second tank port T2 are blocked so as to check flowof hydraulic oil.

[0043] When the change-over valve 51 is set for lowering the boom, thebottom port CB is connected to the second tank port T2 through a firstoil path 41 with a first restrictor 61, and the rod port CR is connectedto the second pump port P2 through a second oil path 42 with a secondrestrictor 62. Meanwhile, a third oil path 43 with a third restrictor 63connects the first pump port P1 to the first tank port T1.

[0044] During the shift of the boom change-over valve 51 from theneutral position to the full boom down position, the open area variationof the restrictors of the respective oil paths 41, 42 and 43 aredetermined as follows:

[0045] As a graph (a2) in FIG. 4, the open area variation of therestrictor 61 of the first oil path 41 is gradually widened so as tomoderate the gravitationally falling speed of the work. To preventhydraulic pressure from pushing the piston of the boom cylinder, theopen area of the restrictor according to the present invention is set tobe larger than the open area of the conventional restrictor of the samekind. In the case where the valve is located in the full strokeposition, for example, an open area S2 of the restrictor 61 in thisembodiment is larger than an open area S1 (see FIG. 12) of theconventional restrictor.

[0046] By the restrictor 62 of the second oil path 42, the hydraulicpressure on the rod side, as expressed by a graph (b1) drawn in a dottedline in FIG. 5, is prevented from increasing, preferably being kept tothe limit. The open area thereof is gradually widened, while it is keptsmaller than the open area of the first oil path 41 (see b2). While theconventional relation of open area is that b is above a, the relation ofopen area according to the present embodiment is that a2 is above b2.

[0047] The open area of the third oil path 43 is restricted during theshift from an initial stroke position k1 to a position k2 so as toprevent the boom from rapidly falling. At the full stroke position, theopen area S3 thereof is set to a value for breed so as to prevent thehydraulic pressure in the pump outlet from excessively increasing.

[0048] Also, the breed pressure at the full stroke position is set to beapplied for jacking up.

[0049] In this regard, as shown in FIG. 6, if the crawler-type travelingdevice 1 has inspection, maintenance or the like, the slewing frame isslewed so as to locate the boom 6 in a side position of the travelingdevice 1, and the boom 6 is lowered so as to raise a crawler of thetraveling device 1 opposite to the boom 6. This movement is referred toas jacking up. In this case, excessive breed at the full stroke positionhinders such an increase of hydraulic pressure on the rod side of theboom cylinder as to be applied for jacking up. On the other hand, thegreater breed with the third oil path 43 reduces the pump pressure (c1)so as to enhance the effect for energy saving. However, the breed needsto be restricted for enabling jacking up.

[0050] Therefore, according to the present invention, the amount ofbreed is set to the limit for enabling jacking up when the enginerotates at the lowest idling speed.

[0051] Due to the above structure and setting of open areas, the pumpoutlet pressure as a graph c1 in FIG. 5 is reduced lower than theconventional pump outlet pressure as a graph c0 in FIG. 13, therebyreducing power loss. When the lowered boom is accelerated, suddenshocking acceleration is avoided so as to improve operability.Furthermore, the truck can be jacked up.

[0052] Alternatively, as shown in FIG. 7, a breed amount change-overvalve 34 is provided on a tank oil passage 46 secondarily connected tothe third restrictor 63 through the first tank port P1 so as to reducepower loss.

[0053] The breed amount change-over valve 34 is a two-positionchange-valve having two ports, which has a spool operation portionconnected to an oil path secondarily connected to the rod port CRthrough a pilot oil path 44. The breed amount change-over valve 34 at anormal position is opened for free passage. When the hydraulic pressureon the rod side of the boom cylinder is increased so as to shift thebreed amount change-over valve 34, a restrict 64 reduces the breedamount, thereby increasing the pump pressure so as to be applied forjacking up. The open area of the restrictor 63 in combination with thebreed amount change-over valve 34 is larger than that of the restrictor63 without the breed amount change-over valve 34, as a graph (c2′) drawnin a double dotted line in FIG. 4.

[0054] Due to the increased breed, the pump pressure is reduced so as togreatly reduce power loss, as a graph c3 in FIG. 5. In the case ofjacking up, the hydraulic pressure on the rod side is increased so as toslide the spool of the breed amount change-over valve 34 through thepilot oil path 44, thereby restricting the breed amount and enablingjacking up.

[0055] The boom change-over valve 51 may incorporate the breed amountchange-over valve 34.

[0056] In this regard, as shown in FIGS. 8 and 9, the boom change-overvalve 51 is provided with a spool 70 including an axial valve hole 70 a.A valve element 71 of the breed amount change-over valve 34 and a spring72 for biasing the valve element 71 are inserted into the valve hole 70a, and a fixing bolt 73 is screwed into the valve hole 70 a so as toclose it.

[0057] Oil path holes 74, 75 and 76 are bored through the spool 70 fromthe outer peripheral surface of the spool 70 while intersecting thevalve hole 70 a.

[0058]FIG. 9 illustrates the boom change-over valve 51 set in neutral,wherein pressure oil flows from the pump port P to the tank port T, andthe valve element 71 is biased by the spring 72 so as to separate theoil path hole 75 from the oil path hole 74.

[0059] If the boom change-over valve 51 is shifted to the boom downposition, as shown in FIG. 10, the spool 70 is slid leftward (in thisfigure) so as to let oil flow from the first pump port P1 to the firsttank port T1 through the oil path 63, and simultaneously, the pumppressure increases so that pressure oil from the oil path hole 75 makesthe valve element 71 rightward against the biasing force of the spring72. The effect of sliding of the valve element 71 is equal to the effectof increase of open area so that the breed is increased as a graph in adouble dotted line (c2′) in FIG. 4.

[0060] If the work truck is going to be jacked up, the gravitationalfalling of the working machine 2 is stopped and the rod pressure b1 issubstantially leveled with the pump pressure c1. At this time, as shownin FIG. 11, the valve element 71 is slid for closing the valve by thebiasing force of the spring 72 so as to check the flow of oil from theoil path hole 75 and limit the breed amount, whereby the boom 6 islowered by the rod pressure and enables the work truck to be jacked up.

Industrial Applicability

[0061] According to the present invention, a change-over valve for aboom cylinder of an excavating/slewing work truck according to thepresent invention is so constructed that, when it is set in a boom downposition, a first oil path with a first restrictor connects a bottomside cylinder port to a tank port, a second oil path with a secondrestrictor connects a pump port to a rod side cylinder port, a third oilpath with a third restrictor connects a pump port to a tank port, thefirst restrictor restricts by such an amount as to gravitationally lowera working machine, and the second restrictor restricts by such an amountas not to exceed pressure on the bottom side of the boom cylinder.Therefore, when the boom is operated to be lowered, the boom is freefrom hydraulic pressure for lowering the boom in addition to the gravityof the working machine, thereby preventing shock, improving operability,and reducing power loss. Furthermore, such reduction of power loss canbe made economically by only change of restricted oil path areas in thechange-over valve for the boom cylinder.

[0062] Also, according to the present invention, the third restrictorrestricts by such an amount as to enable jacking up while an engineidles, thereby reducing power loss in jacking up. Thus, maintainabilityof the work truck is ensured by saved energy.

[0063] Also, according to the present invention, a tank oil passageconnected to the third oil path is provided with a breed amountchange-over valve so as to automatically change the breed amount forsaving energy without manual operation in correspondence to a kind ofwork when the boom is operated.

[0064] Also, according to the present invention, the breed amountchange-over valve is provided in a spool of the change-over valve forthe boom so that change of design is facilitated only by exchanging thespool in addition to that the breed amount for casing energy isautomatically changed without manual operation in correspondence to akind of work when the boom is operated.

What is claimed is:
 1. (Amended) A change-over valve for a boom cylinderof an excavating/slewing work truck, comprising: a spool; and a breedamount change-over valve provided in said spool, wherein, when saidchange-over valve for the boom cylinder is set at a boom down position,said breed amount change-over valve changes the amount of flow in an oilpath connecting a pump port to a tank port.
 2. (Amended) A change-overvalve for a boom cylinder of an excavating/slewing work truck accordingto claim 1, wherein said breed amount change-over valve restricts bysuch an amount as to enable the work truck to be jacked up when anengine idles and a boom is lowered in full stroke.
 3. (Amended) Achange-over valve for a boom cylinder of an excavating/slewing worktruck according to claim 1 or 2, wherein, when said change-over valvefor a boom cylinder is set at a bottom down position, a restrictor of anoil path which connects a bottom side cylinder port to a tank portrestricts by such an amount as a work machine may be gravitationallylowered, and a restrictor of an oil path which connects a pump port to arod side cylinder port restricts by such an amount as pressure on therod side does not exceed pressure on the bottom side.
 4. (Deleted)